The U.S. Postal Service has long lived on the financial edge, but it has never been as close to the precipice as it is today: The agency is so low on cash that it will not be able to make a $5.5 billion payment due this month and may have to shut down entirely this winter unless Congress takes emergency action to stabilize its finances.

"Our situation is extremely serious," the postmaster general, Patrick R. Donahoe, said in an interview. "If Congress doesn't act, we will default."

In recent weeks, Donahoe has been pushing a series of painful cost-cutting measures to erase the agency's deficit, which will reach $9.2 billion this fiscal year. They include eliminating Saturday mail delivery, closing up to 3,700 postal locations and laying off 120,000 workers, nearly one-fifth of the agency's work force.

The post office's problems stem from one hard reality: It is getting squeezed on both revenue and costs.

More Information

As any computer user knows, the Internet revolution has led to people and businesses sending far less conventional mail.

At the same time, decades of contractual promises made to unionized workers, including no-layoff clauses, are increasing the post office's costs. Labor represents 80 percent of the agency's expenses, compared with 53 percent at United Parcel Service and 32 percent at FedEx, its two biggest private competitors. Postal workers also receive generous health benefits.

The Senate Homeland Security and Governmental Affairs Committee will hold a hearing on the agency's predicament on Tuesday. So far, feuding Democrats and Republicans in Congress, still smarting from the brawl over the federal debt ceiling, have failed to agree on any solutions. It doesn't help that many of the options for saving the postal service are politically unpalatable.

"The situation is dire," said Thomas R. Carper, the Delaware Democrat who is chairman of the Senate subcommittee that oversees the postal service. "If we do nothing, if we don't react in a smart, appropriate way, the postal service could literally close later this year. That's not the kind of development we need to inject into a weak, uneven economic recovery."